AFAIK nothing related to the interior and body work is swappable between the w140 and the w124... I have an 87 300TD and an S280 w140 (disassembled the interior completely including the evaporator/box... the car exterior is almost bare at this point)

Maybe the steering wheel... seats and rails are of a different size

Engines if the same model/capacity are pretty much standard across platforms... as are the relays...
locking pumps are different but CAN be made to work on the w124

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85 190E 2.3(SOLD)
86 230E (-->300D) sold
87 300D (-->300TD) sold
68 250S w/ a 615 and manual tranny (RIP)
87 300TD (SOLD)
95 S280 "The KRAKEN" (Turbo 2.9 602 transplant) traded
86 190E 2.3... current project